A very precise control of cell proliferation is necessary for the normal development of multicellular organisms. Deregulated, or inappropriate, cell proliferation is the root cause of many human diseases, particularly Cancer. The E2F transcription factor is a key element in the control of cell proliferation. In a role that is conserved from flies to humans, E2F coordinates the cell cycle-dependent expression of hundreds of genes that are necessary for cells to divide and proliferate. The analysis of E2F and RB homologs in Drosophila has provided a very valuable experimental system that complements the more traditional studies of pRB and E2F family members in mammalian cells. These studies give an opportunity to study E2F function in vivo, in the larger context of animal development. Progress in the previous funding period has provided a detailed picture of the functions of the components of this network, and the ways that the activities of the individual components are integrated. The clear perspective on E2F function provided by this system leads us to the conclusion that, for the control cell proliferation, the key component of this network is the transcriptional activator, dE2F1. To identify rate-limiting steps in dE2F1 function we have taken a genetic approach and have discovered a remarkable number of mutant alleles that strongly modify dE2F1-dependent phenotypes in two different tissues. These provide the first glimpse of the spectrum of cellular activities that have a significant impact on E2F-dependent control of cell proliferation in vivo. These interactors provide a unique opportunity to identify new aspects of E2F regulation and function. We propose to extend our analysis of the dE2F/RBF network by studying the biochemical processes that underlie a newly discovered set of functional interactions between dE2F1 and the components of a repressive submodule of the Mediator complex. Starting from the list of genes isolated in the screen we plan to identify and characterize proteins that limit E2F-dependent proliferation, and proteins that are needed for the activation of dE2F1-dependent transcription.